The packaging materials described in the above European Patent Publications include a core layer of polypropylene containing filler distributed in the polypropylene in order to give the packaging material increased rigidity. In such instances, the polypropylene consists of a propylene homopolymer with a melt index of less than 1 according to ASTM (2,16 kg; 230.degree. C.), or an ethylene/propylene copolymer with a melt index of between 0,5 and 5 according to ASTM (2,16 kg; 230.degree. C.). The quantity of filler is between 50 and 80% of the total weight of the core layer.
From a web or from a prefabricated blank of the packaging material, configurationally stable, liquid-tight packaging containers are produced employing modern, rational packaging machines of the type which forms, fills and seals the packages.
From, for example a web, such a configurationally stable, liquid-tight packaging container is produced in that the web is reformed into a tube, by both longitudinal edges of the web being permanently united to one another by so-called heat sealing in a mechanically strong, liquid-tight lap joint or seam. The tube is filled with the desired contents and divided into closed packages by repeated heat sealings of the tube, transversely of the longitudinal axis of the tube and beneath the level of the contents in the tube. The packages are separated from one another by incisions in the transverse sealings and are given the desired geometric, normally parallelepipedic, final configuration by a further forming and heat sealing operation, for the creation of the finished packaging containers.
From the packaging material, configurationally stable, liquid-tight packaging containers can also be produced by so-called thermoforming which takes as it point of departure the fact that the packaging material is, by heat and pressure (or heat and vacuum), formed into bowl or cup-shaped containers which are filled with the desired contents and thereafter sealed with the aid of a thermoplastic foil which is applied over the openings of the containers and is welded in place by heat sealing around the entire circumferential contour of the opening of the container.
It is also possible to produce such packaging containers in the form of, for instance, bottles by a blow molding technique in a per se known manner.
Packaging containers of the type described above are often employed for packing and transporting liquid or solid foods such as milk, juice, and margarine, and the demand placed on such packaging containers is that they must be configurationally stable and mechanically strong in order to be able to be handled conveniently during transport and use. This means that the packaging containers must withstand impacts, jolts and other external mechanical action to which the packaging containers are subjected under normal handling conditions during transport.
A liquid-filled, closed packaging container of a packaging material known in the art consisting of a propylene homopolymer of a melt index of less than 1 according to ASTM (2,16 kg; 230.degree. C.) and filler admixed into the propylene homopolymer in the above-disclosed quantity possesses good mechanical strength both as regards configurational stability and sealing strength when being handled at room temperature, but functions less satisfactorily at temperatures below 10.degree. C., which often prevail on the distribution of milk. At these lower temperatures, the packaging material becomes brittle and ruptures or cracks easily in the sealing joints or seams when they are subjected to impact and jolts. A certain improvement is, however, achieved by a packaging material consisting of an ethylene/propylene copolymer of a melt index of between 0,5 and 5 according to ASTM (2,16 kg; 230.degree. C.) and a filler admixed into the ethylene/propylene copolymer in the disclosed quantity.
According to the present invention, it has proved that a liquid-tight packaging container of the above-described type can be produced, with even further improved configurational stability and mechanical strength, by a simple modification of the known packaging material.